Thermal printing devices of the general type with which the present invention is particularly but not exclusively concerned are known. They operate with a supply of tape arranged to receive an image and a means for transferring an image onto the tape. In one known device, a tape holding case holds a supply of image receiving tape and a supply of an image transfer ribbon, the image receiving tape and the transfer ribbon being passed in overlap through a printing zone of the printing device. At the print zone, a thermal print head cooperates with a platen to transfer an image from the transfer ribbon to the tape. A printing device operating with a tape holding case of this type is described for example in EP-A-0267890 (Varitronics, Inc.). Other printing devices have been made in which letters are transferred to an image receiving tape by a dry lettering or dry film impression process. In all of these printing devices, the construction of the image receiving tape is substantially the same. That is, it comprises an upper layer for receiving an image which is secured to a releaseable backing layer by a layer of adhesive.
Once an image or message has been printed on the tape, it is desired to cut off that portion of the tape to enable it to be used as a label. For this purpose, it is necessary to remove the releaseable backing layer from the upper layer to enable the upper layer to be secured to a surface by means of the adhesive layer. With existing printing devices, it is difficult to remove the releaseable backing layer from the upper layer: it is necessary first to separate the closely adhered end portions of the releaseable backing layer and the upper layer, for example using a fingernail or tweezers so that the separated end portion of the releaseable backing layer can be finger gripped to peel it off the adhesive layer. This is a relatively difficult procedure and furthermore can result in the ends of the label being damaged in the process.
There have been several attempts to solve this problem. One approach is to provide a so-called tab cut. In these devices, a first cut is made completely through all the layers of the tape to cut off a portion of the tape and at the same time a cut is made through only one layer of the tape. This provides a "tab" which, in theory, can be peeled away reasonably easily. In embossing label makers a system is known whereby a sharp steel blade is used to cut the plastic label against a soft, serrated anvil, such that the soft plastic backing layer remains substantially intact. Such a system does not work satisfactorily with electronic label makers as the backing layer used is normally paper and the plastic label tape is thinner than normal embossing tape. Any attempt to use a similar approach in an electronic label printer would require high cutting forces and frequent replacement of the soft cutting anvil. Although there have been several proposals, no such tab cut has successfully been implemented in a thermal printing device. By way of example, reference is made to EP-A-0319209 which describes one attempt to form a tab cut system. In EP-A-319209, the tab cut is made only through the thickness of the backing layer which is applied as a release layer of double-sided adhesive tape.
In that system, two blades are provided on a cutter support, the blades having different heights so that they penetrate the backing layer to different extents. In this way, one blade cuts through all the layers of the tape at one location while the other blade cuts only through the releaseable backing layer.
One problem which arises with the tab cutting apparatus described in EP-A-0319209 is the control of the height of the blades to ensure that there is reliability in that one blade always cuts through the whole tape and the other blade only cuts through the backing layer. This is difficult to achieve where tapes of differing thicknesses are provided for use with the cutting apparatus. A variation in thickness such that could arise due to normal manufacturing tolerances could even give rise to problems in this respect.
Another difficulty is that the tab cut depends on making two cuts simultaneously from the common cutter support, requiring increased force to be applied by the user. The force is applied manually and the force applied by some users may be insufficient to provide a proper tab cut, causing the label to be damaged when the backing is removed. Conversely too great a force may cause both tapes to be fully cut in both positions, leaving a portion of material within the cutting mechanism.
These problems have meant that to date the above described system has not been successfully implemented.
In this regard reference is made to copending U.S. Application Ser. No. 08/069,256, now U.S. Pat. No. 5,458,423 in the name of the present applicant which describes a tape cutting apparatus in which a tab cut is implemented using a drive means to provide the force for the tab cutting blade. The contents of that Application are incorporated herein by reference. When a motor is used to drive the tab cutting blade, the size of motor is limited by the size of the printing apparatus and so a very high gear ratio is required for a small motor to generate the force required to produce a tab cut, some 15 kilogram force. Due to the high gear ratio, the mechanical inertia in the system will be such that the motor will continue to rotate even when a supply to the motor has been removed. The motor is a small D.C. motor having a wound rotor and a permanent magnet stator. When the current supply to the rotor windings is removed, the rotor ceases to be driven by interaction with the magnetic field of the permanent magnet. However, residual mechanical inertia will mean that the rotor continues to rotate even in the absence of magnetic drive. Thus, as the motor effectively controls the position of the tab cutting blade in relation to the tape, if the motor overruns after a complete tab cut has been implemented, the tab cutting blade will end up in a position in relation to the blade which could cause jamming of the tape. The present invention has been developed to overcome this problem.